This five-year project continues our long interest in the developmental anatomy and biophysics of early heart form and function, with the long-range goal of elucidating mechanisms of congenital malformations such as ventricular septal defects. The proposed experiments and detailed anatomical studies build directly from our mapping of restricted myocyte proliferation as a morphogenetic influence in the embryonic chick heart and in physical conditioning as a mechanical influence in this process. Specifically, this project extends recent focus upon early conduction tissue, as a prominently slow-growing (hard working) network of differentiated muscle, to examine 1) apparent interplay between conduction tissues and immigrating neural crest derivatives in induced ventricular septal defects (VSD), 2) physical factors underlying myocyte conditioning, differentiation of conduction tissue and interactions with other tissues, and 3) similarities and differences in these processes in developing mammals. Aim 1) Mechanisms of ventricular septal defect in chick. Kinetics of key cell populations and valve positioning in widely divergent chick models of induced VSD will be compared under the hypothesis that common forms of that defect spectrum will share common deviations in those indices. Aim 2) Potential physical influences on early development and septation will be modeled and tested in artificial cardiac myotubes and short-term cultures of micro,dissected explants; Similar challenges will be studied in isolated perfused hearts and in an in ovo model of cardiac hypertrophy, In vitro gel collapse studies have now been deferred. Aim 3) Correlative studies in mammals: a) Selected experiments in embryonic and neonatal rats are designed to examine: i) The time course of conduction tissue differentiation and maturation into adult it) Kinetics of tissue movements, cell division and differentiation at intermediate stages of outlet septation. iii) Neonatal aortic banding, to monitor plasticity of conduction tissues in hypertrophic challenge. b) Descriptive studies in rats and in human embryo collections at Washington and London. Reconstructions of outflow structures from series of embryos will be compared for apparent rotation of valves sites and disappearance, through apoptosis or transdifferentiation, of outlet musculature.